Certain naturally occurring carbon containing solids such as oil shale, tar sands, and diatomaceous earth may be retorted to yield an oil useful for producing petroleum products.
Following the pryolysis of the particulate carbon containing solids to extract the volatile components, such as the oil and hydrocarbon gases, a solid material remains which is referred to as "retorted carbon containing solids". This material contains residual carbonaceous material which may be burned to yield heat energy. The heat recovered from this residual carbonaceous material may be used to supply heat for the pyrolysis of the fresh carbon containing solids in the retorting process.
The inorganic residue that remains after the combustion of the retorted carbon containing solids is called "ash". This material is recycled in some retorting processes as "heat carrier material", i.e., the hot ash from the combustor is mixed with raw carbon containing solids and the heat provided is used for retorting the raw material. See for example U.S. Pat. No. 4,199,432. In processes such as this the maximum size of the particles leaving the retort vessel is usually about 0.5 inch or smaller.
In the case of retorted oil shale, during combustion of the residual carbon to produce heat, the physical integrity of the shale particles is changed, and a substantial amount of fine grained burned shale is produced which is not suitable for use as recycled heat carrier particles because such fine particles would be entrained by the product vapor in the retort. Therefore, it is necessary to separate this fine material prior to recycling the coarser grained particles.
In process schemes using a liftpipe combustor to burn the residual carbonaceous material in the retorted carbon containing solids sufficient residence time is required to assure adequate heat transfer between the hot burning particles and the cooler substantially inert heat carrier particles and suitable conversion of the fuel to environmentally acceptable combustion products. In order to achieve the required residence time in the combustion zone a long liftpipe is usually required.
The present invention is directed to an efficient process for burning the particulate retorted carbon containing solids, especially retorted oil shale, and for separating the fine particles of burned shale prior to recycling a coarse fraction of the burned shale back into the retort process.